Light-emitting elements are being required to demonstrate improved luminous efficiency and brightness. There are two methods for improving the luminous efficiency of light-emitting elements, consisting of improving the generation efficiency of light when electrical energy is converted to light within the element (internal quantum efficiency) and improving the efficiency at which the generated light is efficiently extracted outside the element (light extraction efficiency, external quantum efficiency). Considerable research on internal quantum efficiency has resulted in tremendous improvement in recent years. However, since light-emitting element materials typically cause light to be trapped within the element due to such factors as high refractive index and total internal reflection, the light extraction efficiency by which light generated within the element is extracted outside the element is extremely low.
Consequently, attempts have been made to prevent reflection by creating a refractive index gradient by providing nanometer level irregularities on the surface of light-emitting elements as well as extract primary diffracted light by creating a diffraction grating on the surface. However, since these measures require extremely minute processing on the nanometer level, the use of electron beam lithography has been studied at the research level while nano imprinting has been examined for volume production. However, these methods have the shortcoming of requiring the use of costly equipment, while also encountering production difficulty due to the need to fabricate regular structures of nanometer size.
In addition, although known examples of technologies used to roughen a light-emitting surface include technologies consisting of roughening the surface by treating with hydrochloric acid, sulfuric acid, hydrogen peroxide or a mixture thereof (refer to Patent Document 1 and Patent Document 2), since these methods have an effect on crystallinity of the substrate, some surfaces cannot be roughened depending on the exposed orientation. Consequently, since a light-emitting surface cannot always be roughened, there are limitations on the improvement of light extraction efficiency.